Pulley issue?

I am experiencing a loud screech coming from the rear of my 07 Tarmac.
This only occurs when I am descending in excess of 30 miles an hour.
I noticed yesterday that when this occurs my chain looses all tension and ends up laying against my chain stay. Thus, I shift to a low rear gear to coast and when I need to pedal shift up.
I had my ksyrium elite rear hub serviced after this first happened but I think I misdiagnosed the cause since it started occurring again.

too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

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I strongly doubt that it's a pulley related problem. Any friction in the pulleys would only serve to keep the chain in the lower loop, the exact oposite situation to yours.

Usually when the chains spools into the upper loop, and slops onto the chainstay when coasting, it's a freehub problem. There's excess internal friction, and the freehub isn't truly free so the spinning wheel pushes it forward. Often it also shows up as the pedals turning when you walk the bike.

I'd recheck the freehub for sources of drag, and also consider the choice of lube used. An overly viscous lube (except in the ball bearings area) creates excess viscous drag, or in the case of a spinning wheel, viscous propulsion similar to what happens in the torque converter of an automatic auto transmission.

BTW- if it was recently serviced, and the same folks still can't solve the problem, consider getting an opinion from a different mechanic.